<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    *{margin:0;padding:0;}
    #test{
        width:100px;
        height:100px;
        background: paleturquoise;
        padding:15px;
        border:3px solid #333;
    }
    </style>
</head>
<body>
    <div id="test">
        
    </div>
    <script>
    var test=document.getElementById("test");
    var cw=test.clientWidth;
    console.log(cw);
    test.clientWidth=200;
    </script>
</body>
</html>